1. Field of the Invention
The present invention relates to a fixing apparatus used with electrophotographic systems such as a copying machine, electrophotographic printer and the like and with image forming systems such as an electrostatic recording apparatus and the like, for fixing or fusing a non-fixed toner image formed on a recording sheet material, and more particularly, it relates to a fixing apparatus for fixing a non-fixed toner image onto a recording sheet material by pinching and conveying the recording sheet material bearing the non-fixed toner image thereon by means of a pair of rotary members.
2. Related Background Art
In the past, as fixing apparatuses used with electrophotographic systems or with image forming systems such as an electrostatic recording apparatus, a fixing apparatus of roller type comprising a fixing roller incorporating a heating source such as a halogen heater therein and a pressure roller urged against the fixing roller to be rotated therewith and having an outer elastic layer made of silicone rubber has generally been utilized. Such a fixing apparatus of roller type has widely been put in practical use, since it is better than other fixing apparatuses in view of stable conveyance of a recording sheet and/or good security against the firing due to the overheat of the heater.
However, in the above mentioned conventional fixing apparatus of roller type, there arose a problem that, since the non-fixed toner image directly contacted the outer surface of the roller, a portion of the non-fixed toner was transferred to the roller surface to cause a so-called offset phenomenon.
In general, the offset phenomena generated in the above-mentioned fixing apparatus are grouped into two, i.e., thermal offset (offset due to temperature) and electrostatic offset (offset due to electrostatics).
The former includes a low temperature offset wherein the temperature of the toner in the fixing apparatus is too low to be fixed onto the recording sheet, thus causing the offset, and a high temperature offset wherein the toner temperature in the apparatus is high sufficient to melt the toner to a liquid form, thus causing the offset. These offset phenomena can be avoided by properly determining the temperature of the fixing roller by means of a temperature controlling means such as a thermistor on the basis of the feature of the developer (toner), conveying speed of the recording sheet and roller pressure.
On the other hand, the latter, i.e., electrostatic offset may be caused by the fact that the charged toner on the recording sheet is transferred onto the fixing roller electrostatically. Since this offset phenomenon largely depends upon the kind of the recording sheet and circumferential condition, it is difficult to control the transfer of the toner (from the recording sheet to the fixing roller). Up to date, rather than performing the control for directly decreasing the electric field which may cause the electrostatic offset, the disadvantage due to the electrostatic offset, such as the jamming of the recording sheet which may be caused by the smudge of the recording sheet and/or fixing roller due to the offset toner has been partly eliminated by removing the offset toner transferred to the fixing roller by means of a cleaning means such as a felt pad or cleaning blade pressed against the fixing roller. Therefore, the electrostatic offset problem has not yet still been solved.
Further, as well as the provision of the cleaning means, there has been proposed a means for positively preventing the offset of the toner by forming the roller surface with toner-separable material or by applying the separating agent such as silicone oil to the roller surface. However, the electrostatic offset problem has not yet still been solved.
Furthermore, in order to reduce the toner offset, it is also known to float the fixing roller without grounding electrically (referred to as "floating condition" hereinafter). In the case where the fixing roller is electrically held in the floating condition, when a certain amount of toner is transferred or offset to the fixing roller, the fixing roller is charged to the same charge polarity as that of the charged toner, whereby the toner on the recording paper is repulsed from the fixing roller, thus reducing the toner offset.
However, if the fixing roller is electrically held in the floating condition, particularly in the low moisture circumstances, the fixing roller is charged to have a value of a few KV due to the friction with the recording paper, thus causing the discharging phenomenon, which may generate the electric noise leading to the erroneous operation of the image forming system.